1. Field of the Invention
The present invention relates to position sensors or encoders and to apparatus for use in such sensors or encoders. The invention has particular but not exclusive relevance to non-contact rotary and linear position encoders. Some embodiments of the invention are suitable for use in relatively small systems which operate at relatively high temperatures and in which there may be magnetic and electromagnetic interference. Other embodiments are suitable for use in systems having a large measurement range which require a sensor head which is relatively insensitive to mechanical misalignments, dirt, grease and the like.
2. Description of Related Art
Inductive position encoders are well known in the art and typically comprise a movable member, whose position is related to the machine about which position or motion information is desired, and a stationary member which is inductively coupled to the moving member. The stationary member typically includes a number of detectors which provide electrical output signals which can be processed to provide an indication of the position, direction, speed and/or acceleration of the movable member and hence for those of the related machine.
Some of these inductive position encoders employ an AC magnetic field generator mounted on the movable member and one or more sensor windings mounted on the stationary member. The magnetic field generator and the sensor windings are arranged so that the magnetic coupling between them varies with the position of the movable member relative to the stationary member. As a result, an output signal is obtained from each sensor winding which continuously varies with the position of the movable member.
The main limitation of this type of inductive position encoder is that the technique can only work if the space between the magnetic field generator and the sensor windings is free from metallic walls which cannot be penetrated by the AC magnetic field. This therefore limits the applications to which such inductive position sensors can be used.
Another type of inductive position sensor that is known uses permanent magnets and Hall Effect detectors. The problem with such Hall Effect detectors is that they are point magnetic field detectors which can only detect the presence or absence of the permanent magnet. Therefore, such Hall Effect systems do not provide a “continuous” output signal which varies with the position of the movable object.
Another type of inductive position sensor is a Flux Gate sensor which employs a film of soft magnetisable material. With this type of sensor, an AC excitation magnetic field is applied along the plane of the soft magnetisable material causing the material to be driven into and out of magnetic saturation. A DC magnet is usually provided on the movable member, the magnetic field of which interacts with the saturable magnetic element to cause some of the AC in-plane flux to be expelled from the material adjacent the magnet. A number of sensor windings are provided adjacent the material for detecting the flux that is expelled from the saturable element (which depends on the position of the magnet), to output a signal which depends on the position of the magnet relative to the sensor windings. As a result of the driving of the saturable magnetic element into and out of saturation, the output signals that are generated in such Flux Gate sensors are at twice the frequency of the AC excitation frequency. The applicant's earlier international application WO 97/14935 describes such a Flux Gate position sensor. The problem with such Flux Gate sensors is that they are relatively bulky as they employ a relatively large ferromagnetic core around which the excitation and sensor windings are typically wound. They also tend not to be as rugged as smaller more integrated sensor technologies such as the Hall Effect systems discussed above.